Abstract: In selective cooperative relaying only a single relay out of the set ofavailable relays is activated, hence the available power and bandwidthresources are efficiently utilized. However, implementing selective cooperativerelaying in time-varying channels may cause frequent relay switchings thatdeteriorate the overall performance. In this paper, we study the rate at whicha relay switching occurs in selective cooperative relaying applications intime-varying fading channels. In particular, we derive closed-form expressionsfor the relay switching rate measured in Hz for opportunistic relaying ORand distributed switch and stay combining DSSC. Additionally, expressions forthe average relay activation time for both of the considered schemes are alsoprovided, reflecting the average time that a selected relay remains activeuntil a switching occurs. Numerical results manifest that DSSC yieldsconsiderably lower relay switching rates than OR, along with larger averagerelay activation times, rendering it a better candidate for implementation ofrelay selection in fast fading environments.